Modification of RbTiOPO4 (001) crystal surface induced by Ar+ ion bombardment: X-ray photoelectron spectroscopy and first-principles studies

The (001) crystal surface of RbTiOPO4 (RTP) were bombarded by Ar+ ion during different times (2-20 min) and the resulting chemical modification on surface has been studied by X-ray photoelectron spectroscopy (XPS) and first-principles calculations. Ion bombardment can lead to great structural modification on the surface of RTP crystal. Reduction of Ti4+ ions to the lower valence state (Ti3+) and formation of oxygen vacancies occur with Ar+ ion irradiation. With the increase of irradiation time, proportion of reduced Ti3+ ions and number of oxygen vacancies increase accordingly. Fraction of oxygen in P-O-Ti bonds decreases with increasing oxygen vacancies after bombardment for 5-20 min. We think that oxygen vacancies come from breaking of P-O-Ti bonds. The results were also confirmed theoretically by first-principles calculations. Existence of oxygen vacancies is also confirmed by absorption spectra. Formation of Ti3+ state can be ascribed to the loss of surrounding oxygen caused by Ar+ ion irradiation. This work helps us to better understand the ion-beam interaction with RTP surfaces.